The present invention relates to an air-fuel ratio feedback control apparatus of an internal combustion engine and a method thereof, in particular, to technology for feedback controlling an air-fuel ratio of a combustion mixture using a sliding mode control to a target air-fuel ratio.
One conventional feedback control of an air-fuel ratio using a sliding mode control has been proposed in Japanese Unexamined Patent Publication No. 8-232713.
Also, Japanese Unexamined Patent Publication No. 9-274504 discloses a construction that a convergence response characteristic and a convergence stability to an equilibrium point (a target air-fuel ratio) are both obtained by changing an inclination of a hyper plane (a switching line) in accordance with convergence states to the hyper plane (the switching line).
To be specific, in a state substantially converged to the hyper plane (the switching line), the inclination is changed to increase, while in a non-convergence state to the hyper plane (the switching line), the inclination is changed to reduce.
When an air-fuel ratio is feedback controlled, it is common to detect an actual air-fuel ratio based upon an oxygen concentration in the exhaust gas. However, in this case, a delay in an air-fuel ratio detection occurs due to a transport delay of exhaust gas, and when such a detection delay is large, the convergence stability to the hyper plane (the switching line) is deteriorated.
Since, in the air-fuel ratio feedback control disclosed in Japanese Unexamined Patent Publication No. 9-274504, the inclination is changed after the non-convergence state to the hyper plane (the switching line) is judged, a change in inclination is delayed. Thus, there are cases that the convergence response characteristic and the convergence stability to the target air-fuel ratio in the air-fuel ratio feedback control are not obtained at a high level.
The present invention, in view of the foregoing problems, has been achieved and has an object of providing an air-fuel ratio feedback control apparatus and a method thereof using a sliding mode control wherein a convergence response characteristic and a convergence stability are always both obtained at a high level by setting a hyper plane (a switching line) of an appropriate inclination in accordance with a delay in air-fuel ratio detection even when the detection delay is changed due to a change in operating conditions.
In order to achieve the above object, with the present invention, the construction is such that an actual air-fuel ratio is feedback controlled to a target air-fuel ratio using a sliding mode control which restrains the air-fuel ratio state on the switching line set on a phase plane shown by a deviation between the actual air-fuel ratio and a target air-fuel ratio and by a differential value of the deviation wherein an inclination of the switching line is changed corresponding to a change in a dead time of a feedback control based upon engine operating conditions.
According to this construction, the switching line is set on the phase plane shown by the deviation between the actual air-fuel ratio and the target air-fuel ratio, and the differential value of the deviation. The air-fuel ratio is restrained on the switching line to be feedback controlled so as to approach an origin (the target air-fuel ratio), and the inclination of the switching line is changed corresponding to the dead time of feedback control in accordance with the engine operating conditions.
Accordingly, the inclination of the switching line is set in advance to an appropriate value corresponding to the dead time, the convergence stability and the convergence response characteristic to the target air-fuel ratio are improved, thereby capable of reducing a transient error of air-fuel ratio.
Here, the dead time may be set as a detection delay time of air-fuel ratio so that the inclination of the switching line may be set in accordance with the engine operating conditions participating in the detection delay time.
According to this construction, when an air-fuel ratio is detected based upon an oxygen concentration in the exhaust gas, since the detection delay time of air-fuel ratio is changed in accordance with the engine operating conditions, an engine operating condition affecting the detection delay time of air-fuel ratio is detected and the inclination of the switching line is changed in accordance with the detected engine operating condition.
The engine operating condition participating in the detection delay time of air-fuel ratio can be set as an engine intake air quantity so that the inclination of the switching line can be set in accordance with the intake air quantity. Further, it is preferable that the inclination set in accordance with the intake air quantity is corrected in accordance with a differential value of the intake air quantity.
Other objects and features of the present invention will be understood from the following description with reference to the accompanying drawings.